Monday, 23 February 2015

Like many others around the globe I spent yesterday attempting to receive and decode the SSTV transmissions being broadcast from the International Space Station by the Russian Cosmonauts.

The SSTV activity had been due to last three days starting on Saturday but commencement was delayed by the NASA space walk.

Receiving the signal and decoding is relatively straightforward due to relatively high power used (around 25W) however getting a perfect image is a challenge and dependant on a number of factors.

The timing of the overhead pass. Due to the time taken to transmit the image and the three minute delay between each image it is possible to only be in reception range for the end of one image and the start of the next.

The ISS is moving quickly and so the transmission suffers noticeable Doppler shift. FM is more immune to the effect but for optimal performance adjustment of the tuned frequency is required especially on high elevation passes (more information).

The ISS moves position, both in direction and elevation as it moves across the sky and will show up the peaks and troughs in a static antennas radiation pattern. This leads to bands of noise when the signal level falls. The use of a rotatable (and if possible tiltable) antenna (or even an handheld one) is the dirigour mode of operating satellites (and the ISS) for serious enthusiasts.

Noise and local interference will also obviously affect the image.

Mission Control

I opted a two pronged approach, the Yaesu FT857D connected to my rotatable four element YAGI which is mounted horizontally for SSB and the old TRIO/KENWOOD TR9000 was connected to the X50 dual-band collinear mounted vertically.

I had two copies of the MMSSTV program running on separate laptops The TR9000 was left running largely unattended tuned to 145.800MHz, while the FT857D was tweaked to the optimum frequency while the YAGI antenna was rotated to the correct azimuth during the pass.

All adjustments were done manually and I use the Orbitron program for prediction and under the Rotor/Radio tab the frequency and azimuth are shown and updated during the pass (as can be seen in the screen show below)

I missed the first low elevation at 11:07UTC, but was able to monitor and decode images on all the remaining passes during the day, with some excellent results, the images show the full images decodes on both radio set ups as a comparison.

What was slightly worrying and it also happened during the last SSTV activities were some operators transmitting on the downlink frequency even during a pass, what sounded like someone keying up was responsible for the single noise line on another perfect image. I even received an unexpected SSTV image, complete with a call sign while the system was waiting for the next pass. I won't publish it here as everyone makes mistakes.

The experiments are continuing today but I am in work so will just leave an automated set up running on the collinear.

Judging by the messages on social media these SSTV activities seem to have captured the imagination of a lot of operators and several members of my local club South Kesteven Amateur Radio Society (SKARS) had their first go with some excellent results and are hooked! The images can seen on the SKARS Facebook page

Long may the activities continue, hopefully started to transmit some live images from space.

The 70cm band is under used locally and activity seems largely restricted to repeaters. Due to it's vintage the TR9500 doesn't have CTCSS tones and so cannot be used to access repeaters without some modification and I've been looking at adding a CTCSS board.

In the meantime I really wanted to use the TR9500 a bit more and was hoping to make it part of a satellite station, the TR9500 acting as the UHF uplink transmitter (LSB) and the VHF 2m TR9000 as the downlink receiver (USB) for the AO73 (FUNCube-1) and other satellites.

The satellite portion of the band plan is at 435-438MHz and it was when setting this up I discovered the TR9500 neither received or transmitted in the upper part of the 70cm band (435-440MHz) below this everything was hunky-dory.

It hasn't taken long to locate the issue, the HET unit employs two crystals L33 (36.6222MHz), L34 (37.1777MHz) which are switched in to the oscillator Q1 depending on the selected frequency. L33 being referred to as low band, L34 as high band the switching occurring around 435MHz.

The switching HL signal (via R10) and transistors Q3/Q4 are working correctly it is just crystal L34 is not resonating. The surrounding diodes, capacitors, inductors and resistors all look fine, no obvious shorts or broken joints.

I have to do some more diagnostics to rule out any of the passive components but if it is the case that the crystal has failed then it may prove difficult to source an economical replacement.